The concept of multiplexing a hydraulic or pneumatic control to multiple hydraulic or pneumatic actuators or valves is not broadly new. There are several such multiplexed fluid control systems, and this invention would represent a significant advantage in connection with them. U.S. Pat. Nos. 4,913,032, 4,966,065, 4,984,505, 5,048,394, 5,054,369, 5,081,903, and 5,088,383 all assigned to the present assignee, and 4,986,305 assigned to General Electric Company show forms of multiplexed fluid control systems, all of which are hereby incorporated by reference. As exemplified by these aforementioned patents, multiplexed fluid systems typically comprise a modulating valve for producing a controlled output flow and a multiplexer for distributing the controlled output flow to a plurality of second stage valves and/or actuators.
Although other applications exist, one exemplary and very significant application of multiplexed hydraulic control systems is for gas turbine engines, both land based and aircraft types. Gas turbine engines commonly use multiple hydraulic actuators for adjusting their mechanical variables. Mechanical variables can include air valves, fuel valves, hydraulic system pressures, fluid flow rates, engine variable geometry and the like. As engine designers attempt greater performance from the gas turbine engine, the number of mechanical variables desired to be controlled has increased. Applicant is aware of a current desire to variably meter or throttle fuel to individual nozzles or groups of nozzles in a gas turbine engine combustion chamber.
Significant cost and weight advantages exist in using hydraulically multiplexed control systems as opposed to separate hydraulic control systems in which a dedicated hydraulic control for each mechanical variable is necessary. However to realize the cost and weight advantages, it is typically necessary for hydraulically multiplexed control systems to match or exceed the performance achieved by non-multiplexed hydraulic control systems. To provide similar performance as non-multiplexed systems, multiplexed systems need to have precise and reliable closed loop control for variably positioned actuators. Heretofore, closed loop control for hydraulic multiplexed control systems has been provided by a plurality of position sensors and/or potentiometers connected to each variably positioned actuator. By using multiple position sensors, the position of each actuator has been known at all times during operation which provides for reliable control. However, a significant drawback with position sensors has been that they require an undesirable number of components and feedback wiring to provide closed loop control which can undesirably increase costs, increase weight, decrease reliability, and take up valuable space.